Alcohol, such as ethanol and methanol, or mixed fuel of alcohol and gasoline are used as a fuel for an internal combustion engine. Such an alcohol fuel containing alcohol has a lower volatility (combustibleness) than gasoline in low temperature. When the alcohol fuel is used, startability in low temperature and drivability after starting engine may be deteriorated.
As a countermeasure, JP-58-48737A shows a system in which an auxiliary fuel (for example, gasoline) supply apparatus is provided other than a main fuel supply apparatus which supplies an alcohol fuel as a main fuel. When the coolant temperature detected by the coolant temperature sensor is lower than a specified temperature, the auxiliary fuel of which volatility is high is supplied with the main fuel. As the alcohol concentration becomes high, the volatility becomes low. When the coolant temperature detected by the coolant temperature sensor is in low temperature region and the alcohol concentration of the main fuel detected by the alcohol concentration sensor is in high concentration region, the auxiliary fuel is supplied.
The stoichiometric air-fuel ratio of the alcohol fuel is smaller than the stoichiometric air-fuel ration of the gasoline (14.7). For example, the alcohol fuel contains ethanol 100%, the stoichiometric air-fuel ration is 8.9. In a case that the alcohol fuel is used, if the fuel injection control is conducted in the same condition as the gasoline, the fuel injection quantity is excessively increased, so that the exhaust emission and drivability are deteriorated.
As a countermeasure, JP-64-11811B shows the fuel injection quantity is increased by correction according to the alcohol concentration detected by the alcohol concentration sensor. Further, the fuel injection quantity is increased based on the coolant temperature detected by the coolant temperature sensor and the alcohol concentration detected by the alcohol concentration sensor according to a fact that the volatility of the alcohol fuel becomes lower when it is low temperature.
In the auxiliary fuel supply control, if a standard coolant temperature (for example 80° C.) is used as a substitute information of the coolant temperature as a fail-safe for malfunction of a coolant temperature sensor, when the actual coolant temperature is in low temperature region and the alcohol concentration is in high concentration region, the standard coolant temperature is not in the lower temperature region. Thus, the auxiliary fuel supply is not conducted, and the startability and drivability may be deteriorated.
In a fuel injection control, if a standard coolant temperature (for example 80° C.) is used as a substitute information of the coolant temperature, when the actual coolant temperature is in a low temperature region and the alcohol concentration is in high concentration region, an increase coefficient of the fuel injection quantity is established by use of the standard coolant temperature. The fuel increase correction amount is increased and the startability and the drivability may be deteriorated.